Referring to FIG. 1, there is shown an optical encoder system 10 of the prior art comprising light emitter 20 (typically an LED), code wheel or strip 30 having apertures 31a-31f disposed therein, and light detector 40 comprising photodiodes 41a (A) and 41b (A\). In optical encoder 10, collimated light beam 22 emitted by light emitter 20 projects light onto code wheel 60. Collimated light beam 22 is interrupted by masked or optically non-transparent portions disposed between apertures 31a-31f as code wheel 60 rotates in first direction 110 or in second direction 112. (Note that code wheel 60 rotates substantially in a plane defined approximately by collimated light beam 22 as it is projected from light emitter 20 towards light detector 40.) Portions 50a and 50b of collimated light beam 22 project through apertures 31e and 31d onto light detector 40 and sweep across photodiodes 41b (A\) an 41a (A) as code wheel 60 rotates in direction 110 or 112 in the plane. As code wheel 60 moves in direction 110 or 112, the light patterns projected onto first vertical portion 70 of light detector 40 by beam portions 50a and 50b change, and the output signals provided by photodiodes 41a and 41b change correspondingly. These output signals are generally employed to generate a pair of quasi triangular signals, which are then used to determine any one or more of the position, speed and direction of code disk 60.
In practice, optical encoder 10 of the type illustrated in FIG. 1 suffers from several disadvantages, including undesired variations in the amplitudes of the output signals provided by photodiodes 41a and 41b. For example, the amplitudes of the output signals provided by photodiodes 41a and 41b may not be properly synchronized. Undesired amplitude variations such as these may arise due to process variations occurring during the manufacture of optical encoder 10, errors or misalignments in the layouts of the photodiodes 41a and 41b, wobbling of code disk 60, variations in the light output by light emitter 20, mechanical misalignment in or of one or more of light emitter 20, code wheel 60, or light detector 40, and other problems. In an optical encoder 10 of the type shown in FIG. 1, symmetrical amplitudes between output signals provided by photodiodes 41a and 41b are important to permit subsequently performed signal processing techniques to be accomplished accurately.
FIG. 2 shows idealized output signals provided by photodetectors 41a (signal “A”) and 41b (signal “A\”), where maximum and minimum amplitudes 105 and 107 of such respective output signals are achieved, and where the respective maxima and minima of such signals occur at precisely the same times. In actual practice, however, such synchronization of the peaks and valleys of output signals A and A\ may be difficult to achieve owing to the factors described above.
Several approaches have been taken in the prior art to solve the foregoing problems. One such approach is described in U.S. Pat. No. 5,463,393, where two amplitude-encoded sinusoidal signals are squared and summed to provide a compensatory signal. Another such approach is described in U.S. Pat. No. 5,134,404, where quadrature signal errors are continuously corrected by generating an error function signal equal to the sum of squares of the normalized values of each of the quadrature signals, minus a reference constant. Most prior art techniques for correcting errors in quadrature signals fall into one of several categories: matching channel gains to minimize gain error; system calibration, and cross-channel coupling to reduce spatial quadrature errors. To date, none of these solutions to correcting undesired amplitude variations in the output signals provided by photodetectors in optical encoders has proved entirely satisfactory. Additionally, some prior art techniques for correcting such amplitude variations require relatively complicated data processing techniques and circuitry, which of course adds to the cost of an optical encoder.
What is needed is an optical encoder where amplitude variations in the outputs provided by the photodetectors or photodiodes thereof may be corrected quickly, accurately and reliably without resorting to complicated data processing techniques and circuitry, and that may be manufactured at low cost.